Many laser micromachining processes require laser pulses of various shapes. For example, laser processing of multiple layers of different materials demands fairly large adjustable ranges of laser power, pulse repetition rate, pulse width, spot size, and pulse shape to achieve either or both of desired quality and throughput. With a conventional Q-switched laser, all of the parameters can be adjusted over only limited ranges, especially the pulse width and pulse shape. Conventional lasers generally limit the option of having adjustable pulse width and pulse shape. As another example, the processing of electrically conductive links on memory chips or other integrated circuit (IC) chips demands pulses with a fairly short leading edge rise time (e.g., 1 ns-2 ns rise time) to achieve desired quality and yield. With existing solid-state lasers, the leading edge rise time changes with pulse width. A traditional longer leading edge rise time of a 5 ns-10 ns wide laser pulse could result in excess removal of (“over-crater in”) the overlying passivation layer material during link processing, especially when the overlying passivation layer is too thick or varies over a wide range across a wafer or among a group of wafers.
For via drilling of certain printed circuit boards (PCBs), pulse width critically affects throughput and via size. Certain of these drilling applications require laser pulses of sub-nanosecond, or even picosecond, pulse widths. Some laser link processes using traditional laser pulses with pulse widths of a few nanoseconds to a few tens of nanoseconds tend to over-crater in thicker overlying passivation layers and thereby cause IC reliability problems. Use of a laser pulse with a special shape and a fast leading edge, such as tailored pulse, is one technique for controlling link processing. The shaped laser pulse can be generated by diode-seeding a fiber laser and amplifier (MOPA). However, the MOPA structure constructed with a fiber laser as the power amplifier is quite complicated and costly. Moreover, the fiber amplifier suffers from a relatively low intensity damage threshold, which in turn reduces laser reliability and limits the laser pulse intensity available.
On the other hand, conventional active Q-switched solid-state lasers can provide nanosecond-duration pulses with high pulse energy, but they deliver only a traditional laser pulse shape (i.e., typical Gaussian shape), with a leading edge rise time that is close to the laser pulse width itself.
Although there are many techniques for generating laser pulses with very short rise times in the range of 1 ns or shorter, such as passive Q-switch or mode locking techniques, the pulse widths are, however, close to the short rise time. More important, the pulse shape is generally fixed for such laser pulses.